The present invention relates to a radio base station selection method in a mobile communication system. More specifically, the present invention relates to such a radio base station selection method and system in which a radio base station is selected when a mobile station sends or receives a call or performs a handoff in a mobile communication system.
In addition, the present invention relates to a radio base station which can select another radio base station according to such a radio base station selection method.
At present, cellular mobile communication systems such as the PDC (Personal Digital Cellular) system and the PHS (Personal Handy phone System) are operated. In these mobile communication systems, selection of a radio base station when sending/receiving a call or handing off is carried out as explained below.
In the case of PHS, Selection by a mobile station:
When establishing connection for sending/receiving a call in the PHS, a mobile station sends a request for establishing a link channel to a radio base station, in order to get a radio channel allocation. If the radio base station has an available radio channel, it assigns a link channel to the mobile station. On the other hand, if the radio base station does not have an available radio channel, it sends a link channel assignment rejection to the mobile station. After receiving the link channel assignment rejection, the mobile station searches for other radio base stations with an available radio channel. The mobile station repeats the search until it finds a vacant radio channel or it reaches a predetermined repetition time.
In the PHS system, when the quality of communication between a mobile station and a radio base station is degraded (reception quality degradation), a handoff sequence is activated. Then the mobile station searches control frequencies for a control signal from adjacent radio base stations. If the mobile station receives a control signal, it becomes synchronized with the control signal, and sends a request for establishing a link channel to the adjacent radio base station to handoff in a manner similar to the above. The subsequent process is the same as the above.
In the PHS system, 64 kbps data transmission can be obtained by using two time-shared slots. When the radio base station receives the request for establishing a link channel, it assigns the first slot link channel. The mobile station sends an additional request for Tch (communication channel) using the assigned link channel to request an additional slot. If the radio base station has an available radio channel, it assigns one additional channel. If the radio base station has no available radio channels, it sends a Tch additional assignment rejection to the mobile station. The mobile station sends Tch additional requests again to the radio base station, until the request is accepted or it reaches a predetermined repetition time.
In the case of PDC, Selection by a radio base station:
In the PDC network, a radio base station control station is provided above the radio base stations to control radio channel assignment/switching. A mobile station measures received levels of the control channels of adjacent radio base stations. When sending/receiving a call, the mobile station sends a call establishing signal and the received levels. If a radio base station in the service of which the mobile station belongs has no available radio channel, the radio base station control station asks adjacent radio base stations about their channel availabilities based on the received levels reported by the mobile station. If some adjacent radio base stations have channel availabilities, the radio base station control station selects a radio channel and notifies the mobile station of the selection.
A handing off is activated when the received level of the radio communication link is degraded. During spare time in TDMA slots, the mobile station checks the received levels of adjacent radio base stations. The mobile station reports the received levels to the radio base station control station periodically. When the quality of communication link is degraded, the radio base station control station notifies the mobile station of a radio base station to hand off, based on the received levels of the adjacent radio base stations reported from the mobile station.
It is necessary to broaden the bandwidth per channel in order to improve radio transmission speed in the present mobile communication system. However, a high frequency band is needed for assuring a broad frequency bandwidth. In addition, in order to satisfy subscriber capacity required in the system, frequency efficiency should be improved to higher than the conventional system. Judging from these factors, a micro-cell (about 100 m to 300 m) is expected to be used for realizing high speed access in future mobile communication systems.
In such a micro-cell configuration, the number of radio channels covered by one radio base station is usually small, because the service area is covered by many radio channels. Therefore, if many mobile stations come together in a radio zone covered by one radio base station, all the radio channels of the radio base station are easily occupied, and can not provide the required communication service.
Solutions for preventing the above situation includes increasing the number of radio channels per radio base station, and placing multiple radio base stations close to each other to increase the traffic capacity per area. The former solution requires additional hardware in radio base stations and results in high cost, and therefore, the latter solution is usually desired.
In the micro-cell configuration, the number of hand-offs will be increased because of the small cell size. FIG. 8 shows a mobile station traversing micro-cells. In this figure, when the mobile station moves along a path 6-1, it is handed off at locations 6-7, 6-8 and 6-9 to switch radio areas from 6-2 to 6-3, 6-4 and 6-5, respectively. As shown in the figure, the number of hand-offs is increased in the micro-cell system, therefore it is important to shorten hand-off time as much as possible. Therefore, it is important to find an available radio channel as fast as possible. This problem occurs when sending/receiving a call as well as handing off.
In the PHS system as explained above, there is no need for controlling radio base stations. However, searching time to find a radio base station having an available radio channel is very long, leading to many problems.
In the PDC system as explained above, radio channels can be assigned with high speed and high efficiency. However, it needs radio base station control stations above radio base stations, which complicates the network and increases network cost.